A conventionally known illumination apparatus applied to projection exposure apparatus is one using a fly's eye lens as optical integrator. The illumination apparatus with the fly's eye lens arranged therein illuminates the projection master with light in a state in which light beams from secondary light sources formed by respective wavefront division facets of the fly's eye lens are superimposed. For illuminating the projection master with light having a uniform illuminance distribution, there is a known illumination apparatus provided with a correction optical system for correcting an illuminance distribution of a beam entering the fly's eye lens (e.g., cf. Patent Literature 1).
The illumination apparatus of Patent Literature 1 is configured as follows: when the illuminance distribution of the light illuminating the projection master is different in a central region or in a peripheral region of the projection master from the other and when angular intensity distributions of focused beams are different from each other depending upon positions on the projection master, the correction optical system is located between a light source and the fly's eye lens to change illuminance distributions of beams incident into the respective wavefront division facets of the fly's eye lens so as to correct nonuniform angular distributions made in apertures of respective beams focused on the projection master, thereby obtaining a uniform illuminance distribution.